During the production of hydrocarbons from underground formations, it is often desirable to reduce the permeability of certain zones in the formation. This is the case, for example, when a formation next to the production well is likely to direct water into the well. Another example is where there are zones of high permeability next to the injection well.
One method of reducing the permeability of an underground zone entails introducing into the zone a fluid with the intrinsic potential of creating a gel by precipitation of a solid phase. Typically it is necessary to control two parameters, which are the delay required for the formation of the gel, and the morphology of the solid phase that is produced during gelation; the delay determines the possible degree of penetration into the formation, while the morphology of the solid phase determines the degree to which the permeability of the formation will be reduced.
Numerous simple salts of aluminium III (such as aluminium chloride or aluminium nitrate) are soluble in slightly acid mediums, but form insoluble aluminum hydroxides when the pH level goes above about 5. Delayed formation gels based on such hydroxides can be generated from solutions consisting of a suitable aluminium salt and an activator. For example, the activator is used to slowly increase the pH level of the solution in the temperature range normally existing at well bottoms, and this in turn leads to the delayed formation of an insoluble hydroxide phase. A suitable activator is urea, which undergoes hydrolysis at high temperature, with formation of ammonia; the latter in turn increases the pH of the solution. The solid aluminium hydroxide precipitate phase has a tendency to stay amorphous, with the consistency of a gel.
U.S. Pat. No. 4,889,563 describes a composition in which aluminium hydroxychloride is used instead of aluminium chloride, providing various operational advantages. The activator for the composition is urea, urea derivative or hexamethylene-tetramine.
U.S. Pat. No. 3,730,272 also describes inorganic gelling fluids for reducing permeability to modify an injection profile. The fluids comprise polyvalent metal salts that form gelatinous metal hydroxides or hydrated metal oxides of very low water solubility. The patent mentions the use of a range of polyvalent metals including aluminium, chromium, iron, copper and bismuth, such metals being used in the form of water-soluble salts such as chlorides, nitrates or acetates. However, the examples focus on the use of aluminium chloride. A pH-increasing reactant such as urea is used to trigger gelation/precipitation and, for applications in the lower temperature range, a water-soluble nitrite salt is used to accelerate the gelation/precipitation process.
The use in hydrocarbon well applications of gelling systems based on sodium silicate solutions is also known. See for example, P. H. Krumine and S. D. Boyce, “Profile modification and water control with silica gel-based systems”, SPE 13578, 1985; D. D. Sparlin and R. W. Hagen, “Controlling water in producing operations”, World Oil, June 1984; and J. K. Borchardt, “In-situ gelation of silicates in drilling, well completion and oil production”, Colloids and Surfaces, Vol. 63, 189, (1992).
R. S. Seright and J. Liang, “A comparison of different types of blocking agents”, SPE 30120, pp 431-440, reviews issues of placement and permeability reduction in respect of various fluid diversion processes. R. S. Seright, “Improved Techniques for fluid diversion in oil recovery processes”, Second annual report DOE/BC/14880-10, US DOE, February 1995, develops theoretical models relating to the feasibility of using particles to prevent gelant penetration into low permeability zones during placement.